Get test Python version from the HA Core repo.

[ademuri]

This changes the update-test-matrix.py script to fetch the required Python version for a given HA Core version by fetching the pyproject.toml for that version. It updates the test matrix with the updated script. It also updates the Python version used for building the Docker image and devcontainer to 3.14, since that's the version now required by HA Core.

This fixes a CI breakage I saw on a draft pull request:

+ uv pip install -e core/
  × No solution found when resolving dependencies:
  ╰─▶ Because the current Python version (3.13.12) does not satisfy
      Python>=3.14.2 and homeassistant==2026.3.0.dev0 depends on
      Python>=3.14.2, we can conclude that homeassistant==2026.3.0.dev0 cannot
      be used.
      And because only homeassistant==2026.3.0.dev0 is available and you
      require homeassistant, we can conclude that your requirements are
      unsatisfiable.

[basnijholt]

Sorry it took me so long to get to this. I've been a bit overwhelmed by the number of AI-assisted PRs opened here recently, and I've also been spending nearly every spare hour on my biggest project so far, MindRoom.

I'm very supportive of using AI for coding, but many of these PRs still need careful human review because even plausible-looking changes can introduce subtle breakage. That backlog made me postpone reviewing them for a while.

I've now done a batch review with Codex / GPT-5.5 (xhigh). This comment is AI-assisted, but I've reviewed it before posting.

The idea of deriving the test Python version from HA Core is good, but this is not mergeable in its current state.

The current checks include failures for the dev Python job and validate_hassfest. Also, this PR is behind current main, and it overlaps conceptually with later Python-version Renovate updates. Could you rebase, make the generated matrix match current HA Core, and get the full CI matrix green before this is considered again?